1. Field of the Invention
This invention relates to a material capable of forming a coating film adhering with great fastness to the surface of titanium or a titanium alloy (hereinafter referred to as "titanium metal"), a method for the production of the material, a method for coating a titanium metal with the material, and titanium metal article coated with the material. This material is compatible with living tissues.
2. Description of the Prior Art
Since titanium and titanium alloys possess high strength and exhibit no toxicity to living tissues, efforts are being continued to realize their utility as materials for artificial roots of teeth, artificial bones, and artificial joints. The materials using titanium as a main component, however, do not promote osteoinduction, osteoconduction or bonding of new bones because they are inactive relative to living tissues. These materials, therefore, are incapable of being kept buried in living tissues in a stable state for a long time. In contrast, the calcium phosphate compounds such as apatite hydroxide and tricalcium phosphate that are the main components of such living inorganic substances as bones and teeth, exhibit excellent bioaffinity for living tissues in terms of nontoxicity, osteoconduction property, bone bonding property, and bone replacement property. Sintered articles of calcium phosphate compounds, however have low mechanical strength and, therefore, have limited utility. In view of the circumstances described above, a composite material comprising titanium or a titanium alloy as a substrate and a coating of calcium phosphate covering the surface of the substrate and having a property of bioaffinity and high strength is desired.
The conventional methods of coating a materials having titanium as its main component with a calcium phosphate compound are of three types, (1) the flame spraying method, (2) the spattering method, and (3) the dispersing method. The flame spraying method comprises dissolving calcium phosphate powder in a flame or a plasma kept at a high temperature and spraying the dissolved powder at a high speed on the surface of a material formed mainly of titanium. Since this method has to be conducted at high temperature, it often induces decomposition of the calcium phosphate and alteration of the crystalline structure of the compound. The spattering method comprises spattering a calcium phosphate compound onto the surface of a material having titanium as its main component. Since this method requires the spattering in a high degree of vacuum, the productivity is low and the cost of production is high. Moreover, it is difficult to produce a coating layer of a sufficient thickness by this method. The glass dispersing method comprises dispersing a calcium phosphate compound in molten glass, applying the resultant molten glass on the surface of a material comprising titanium as its main component, and solidifying the applied coating by cooling. The coating material prepared by this method is deficient in durability in living tissue because the dissolving speed of glass type material is higher than that of a crystal type material in living tissues.
As described above, each of the conventional methods for coating a surface with calcium phosphate has its defects.
In the circumstances, it is a need to develop a commercially feasible method of coating a titanium metal with calcium phosphate, inexpensively with a simple device.